In laser applications such as laser machining, lithography, and optical inspection, not all wavelengths of interest can be supplied as fundamental radiation from commercially available lasers. In such cases radiation having a desired wavelength is provided by frequency-converting the fundamental radiation in one or more optically nonlinear crystals. Such frequency-conversion includes frequency-multiplication, for example, frequency-doubling tripling or quadrupling, also referred to as harmonic-generation. Frequency conversion also includes sum and difference frequency generation. Frequency conversion further includes parametric conversion, which can be referred to as frequency division, wherein radiation having a first frequency is converted into radiation at two other frequencies, the sum of which is equal to the first frequency.
Frequency-quadrupling is typically accomplished using two frequency-doubling stages. By way of example, fundamental radiation of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at a wavelength of 1064 (nanometers) nm is converted to radiation (second-harmonic radiation) having a wavelength of 532 nm in a first frequency-doubling stage and the 532-nm radiation is converted to (fourth-harmonic) radiation having a wavelength 266 nm in a second stage. Further, by way of example, a lithium borate (LBO) crystal can be used for the second-harmonic conversion and a cesium lithium borate (CLBO) can be used for the fourth harmonic conversion.
Most frequency-conversion processes in an optically nonlinear crystal have a conversion-efficiency, at most, of about 50%. Accordingly, conversion-efficiency in multiple optically nonlinear crystals is significantly less. In the above-discussed example of frequency-quadrupling of 1064-nm radiation from a Nd:YAG laser, conversion-efficiency is between 5% and 25%, for example 40 Watts (W) of optical power at 1064 nm will provide only about 10 W or less of 266-nm radiation. The residual optical power at 1064 nm and 532 nm has to be disposed of (“dumped”), generating heat.
Usually, the crystal or crystals used for frequency conversion, and optical components for focusing radiation into the crystal or crystals, are contained in a hermetically sealed enclosure. Hermetic sealing is useful because the crystals are best protected from environmental contamination.
Different methods have been used to dump residual power. One of these methods is to direct the residual radiation onto a highly scattering surface or bulk material, such as a ceramic, within the enclosure. This scatters the residual radiation in all directions and minimizes the possibility of local heating anywhere in the enclosure. This method is primarily used to dump visible and infrared radiation. Scattered ultraviolet (UV) radiation may cause accelerated photo-degradation of components inside the frequency-conversion enclosure.
Another method is to direct the residual radiation out of the enclosure. This method, however, requires additional optical components to steer the residual radiation within the enclosure and one or more optical windows in the enclosure to allow the radiation to escape. In addition the enclosure must be configured such that there an unimpeded optical path or optical paths to the window or windows.
In yet another method radiation is directed into a radiation-absorbing device or beam-dump within the enclosure. The beam-dump is typically connected to a heat sink that is usually water-cooled or air-cooled. Water-cooling adds complexity and cost, as materials have to be selected for seals and the like that will not generate contaminants within the enclosure. Heat removed by air-cooling is deposited within the enclosure and can heat the enclosure to a temperature at which the conversion efficiency of the crystals is reduced.
There is a need for a simple method and apparatus for disposing of residual radiation power from an enclosure in which frequency-conversion in optically nonlinear crystals is carried out. Preferably, the method should minimize generation of heat within the enclosure.